A payment made by one or more parties in an agreement to other parties, to induce them to join the agreement. Suppose, for example, that in a group of firms each has a profitable plant. They calculate that if one plant were closed and its output shifted to the remaining plants, total costs for the group would be lower and thus total profits would rise. If each firm gets its profits solely by selling the output from its own plant, no agreement can be reached, since the firm whose plant closes loses all its profits. The firms can thus only be induced to agree to a plan which includes side-payments, by which the firms whose plants remain open use part of their increased profits to compensate the firm whose plant closes down.
Reference: Oxford Press Dictonary of Economics, 5th edt.